The top MMA fighters are all from small time boxing, wretling, BJJ or some other place with a strong competitive history. I would say the top athlete out of all of the big guys is Coleman, who once in his life managed to beat Angle.
Wrestling is a tough sport, but it caps out at the Oly level, where as Oly boxing is many many levels below what the pros do.
Anyone who is cabable of boxing as a decent pro and puts their heart into some MMA would destroy, it is just a different class of athlete. I have yet to see one freaking top MMA guy who has even passable punching ability, from a pro point of view, they all plant and punch or run into them, if they had some better skills it would be A LOT more entertaining.
Face it, about 65% of MMA ends up as a snoozer, most of these fighters are not willing or able to take out their opponent with proper puching while shooting, so grapling is the natural choice.